Aircraft: B757
Phase of flight: Descent
Anomaly: ATC Issue All Types; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy

We were flight planned for the LTOWN arrival but ATC rerouted us to ARG for the FNCHR arrival. Approaching ARG from the east; we received radar vectors for spacing which took us to the north and west of ARG at FL240 and 320 KIAS. At a point about 15 NM to the northwest of STANI; we received the following clearance from ATC: 'Cleared direct to STANI; 'descend via' the FNCHR 1 arrival. After we executed the new route in the FMC we noted the 290 KIAS restriction which was appropriate at STANI had not been programmed. This was because we did not cross ARG (the point prior on the arrival where the restriction was charted). Thinking we had to be at 290 KIAS; the pilot flying speed intervened while pilot not flying made the adjustment in the FMS. Pilot not flying set 4;000 FT in the altitude window (final restriction at JAYWA) so as to descend via the FNCHR arrival. The airplane began an immediate descent to make the 'at or below 230' restriction at STANI. This is where I believe an error was made. At the time we left FL 240; we were within 10 miles to the NW of STANI but not actually on the FNCHR arrival. While not on the arrival; we should have asked for an altitude change so as to meet the 'at or below' restriction at STANI.The ATC clearance was incomplete. We should have been given more guidance with regard to altitude. Either a clearance to descend to FL230 or a crossing restriction at STANI would have been appropriate. Being fast and close to STANI did not allow the crew enough time to discuss the issue and there was confusion as to whether we should have crossed STANI at FL240 and then started the descent or initiated a descent so as to meet the restriction at STANI. We were in VNAV PATH and I entered 4;000 FT into the altitude window and then a speed intervention of 290 KIAS to comply with the 290 KIAS at Walnut Ridge; which we were inside of; and to also be in compliance with our next restriction of Below FL230 at 290 KIAS at STANI. We crossed STANI below FL230 and at 290; meeting that restriction. The problem we believe that we made is that we weren't on a published portion of the approach; so therefore we departed an assigned altitude of FL240 to meet the STANI restriction. We should have clarified our clearance with ATC; whether to cross STANI at FL230; or FL240 and what airspeed we wanted. I initially thought he wanted us to go direct to STANI and descend via the FINCHR; meeting the first restriction at STANI; but then we started to doubt that was the clearance and thought perhaps he wanted us to proceed to STANI; cross it at FL240 and then descend VIA. A simple clarification would have solved this issue. During all this; the Controller was very busy with other traffic as many airplanes were getting vectored; speed assignments and rerouted with new STARS. A B757-200 flight crew cleared to fly a radar vector to intercept the FNCHR RNAV STAR inside of ARG questioned whether they should have begun a descent to cross their first waypoint at the charted altitude since they were technically not established on the STAR.